In the heat treat field, metal work is to be heated and cooled in accordance with known, time-temperature-atmosphere composition heat treat processes. Simplistically, the work is heated, held and cooled at specific rates and times while the gaseous or furnace atmosphere surrounding the work is controlled to impart desired metallurgical and mechanical properties to the work. Cooling of the work is physically accomplished in one of two ways.
Typically, a heat exchanger is physically located outside the furnace and air or furnace atmosphere (depending on the heat treat process) which is heated from coming into contact with the hot work is pumped from the furnace through the heat exchanger where it is cooled and then pumped back to the furnace. External heat exchange systems are fundamentally sound. Air infiltration is the major hazard to product quality. All ducts and components must have gas-tight welds and welds which are subjected to severe heating and cooling and must be water cooled, for example by water jackets, to prevent cracking. Thus, the major disadvantages to the external heat exchange systems are higher installation costs, expensive operation and air infiltration. Higher operating costs are due to the need for much larger fans.
To overcome the disadvantages of the external heat exchange systems, Surface Combustion, Inc., the assignee of this invention, developed internal heat exchange tubes initially for application to bell-type coil annealing furnaces. The basic device is disclosed in Cone U.S. Pat. No. 3,140,743 and improved upon in Mayers et al U.S. Pat. No. 4,247,284, both of which are incorporated herein by reference. The internal heat exchange tube marketed by Surface Combustion under the brand name "INTRA-KOOL" has been used in batch-type, industrial heat treat furnaces other than batch coil annealers.
In the internal heat exchange application, a finned tube or pipe is positioned within the furnace with an inlet end outside the furnace and an outlet end also outside the furnace. When the work is to be cooled, a coolant is injected at one end of the tube and the "spent" coolant is recovered at the opposite end. The furnace fan directs the furnace atmosphere over the tubes to establish heat transfer therewith. This cooled atmosphere is then directed by the fan over the work where it is heated from contact therewith and recirculated against the cool tubes, etc.
As discussed in Mayers and in some detail in the Detailed Description of the Invention which follows, if water is the coolant and if water is immediately injected into the tube, high thermal gradients will result in some bending or deformation of the tube and stressing the tube to failure. The problem occurs, as will be explained later, during the initial application of the coolant, i.e. water, in a time frame which can be as short as one-half second and extend to as long as about six seconds. The hot tube vaporizes the water to steam and when the steam barrier is broken by the water plug, circumferential thermal stress gradients occur and bend the tube. Once steady state water flow occurs, the gradients are reduced or eliminated and the tube returns to its original shape. However, the tube is bent. To minimize the problem, the tubes are installed as straight tubes into the furnace with inlet at one end and outlet at the other end. This requires two separate manifolding arrangements for supply and collection of water. Bending the tubes in a circular fashion as shown in the coil annealer prior art patents aggravates the pipe distortion problem.
The short tube life resulting from thermal gradients was addressed in Mayers by injecting initially cool air into the tube followed by increasing amounts of water mist spray prior to injecting the water. Alternatively, water mist spray could be initially injected. The mist spray basically provided for controlled cooling of the tube to a temperature whereat water could be injected without forming the steam barrier. While Mayers addressed and resolved a problem, the cooling rate is necessarily slowed and the temperature gradient, is difficult to control because, in part, steam pockets tend to randomly occur and pipe bending still occurs.